We identified a novel eukaryotic transcriptional control element which markedly increases the transcriptional activity of eukaryotic genes. This element has been termed activator or enhancer. We first characterized the activator in the SV40 genome and in the Moloney sarcoma virus (MSV). We have now been able to localize the core activity to a very short stretch of nucleotides (approximately 10 to 15 bp) within the SV40 72 bp repeat. Similar core sequences have been detected in the enhancer region of polyoma, BK and MSV. We have demonstrated that sequences flanking the coding genes of endogenous retroviral genes of either mouse, monkey or human origin also harbor enhancer activity.